Hi Dave,
interesting problem, with those bolts!
Firstly, I would not believe that they can get so hot from hysteresis
losses. They are not really taking much magnetic flux at all, because
they are separated from the laminations by the air gap made by the
clearance between the bolts and the holes. As a consequence, replacing
these bolts with nonmagnetic ones, such as stainless steel, will not
help, or at least not because of any magnetic phenomena.
The heat must be caused by an enormous current flowing through them, or
more likely, a quite large current flowing through the poor connections
made by the nuts on the surface they sit on. So, the obvious solution is
to insulate them, at least one end. You say that the bolts are thinner
than the holes' diameter. That's great, as it might allow you to use
these insulating washers that also have a sort of little tube attached.
Sorry for the poor description, but I mean insulating washers such as
those used for mounting transistors on heat sinks, using metal bolts.
Complementing the in-hole insulation with a tube made of paper is also a
good idea.
Regarding material of the bolts, what might help is using a material
with poorer conductivity. After all, the best reason why transformer
laminations are made of silicon steel is that this alloy has much higher
resistance than carbon steel! That reduces eddy currents. It is possible
that your problem comes mostly from eddy currents in the bolts, and in
that case the only thing that would help is using a material with higher
resistance. But I don't think this is the case, because of the air gap
you have there, which should keep magnetic flux and thus eddy currents
in the bolts rather low.
If all else fails, use some longer pieces of angle stock so that you can
install the clamping bolts outside the laminations, to keep them free of
strong fields and make them easy to insulate.
I would not use the transformer like it is, because such extreme heating
of the bolts can cause more problems, in addition to being unnecessary
power loss.
Manfred, XQ6FOD.
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